Hitherto, cemented carbides (WC-Co alloys or WC-Co alloys to which carbonitrides of Ti, Ta or Nb are added) have been used as a tool material for cutting metallic materials. However, as cutting speeds have lately been increased, a tendency of using cemented carbide tools comprising cemented carbide substrates coated with coated films consisting of carbides, nitrides, carbonitrides, carboxides, boronitrides or oxides of Group IVa, Va and VIa elements of the Periodic Table or Al or their solid solutions by CVD or PVD methods in a thickness of 3 to 15 .mu.m is enhancing. The thickness of the coated films tends to further increase and CVD coated cemented carbides with a coating thickness of at least 20 .mu.m have been proposed. In such CVD coated cemented carbide tools, there arises a problem that a tensile residual stress occurs in the coated film during cooling after the coating due to difference in coefficient of thermal expansion between the coated film and substrate, and the breakage resistance of the tool is thus lowered.
For a coated cemented carbide tool, on the other hand, it has been proposed in order to improve its breakage resistance, to introduce cracks into a coated film to be penetrated therethrough to a substrate by applying mechanical impact to a surface of a cemented carbide, for example, by blasting (JP-B-7-6066). In this proposed method, it is confirmed that the breakage resistance can be improved to some extent, but because of previously introducing cracks into the coated film to be penetrated therethrough to the substrate, Griffith' precrack length is increased, thus resulting in lowering of the breakage resistance, wear fluctuation of the coated film and deterioration of the wear resistance from the longer cracks.
As described above, the coated cemented carbide tools of the prior art have the problems that when the thickness of a coated film is increased to improve the wear resistance, the breakage resistance of the tool is decreased and even when cracks are previously introduced into a coated film with a relatively large thickness, the wear resistance is rather lowered depending on the cracked state. These problems have not been solved yet.
Under the situation, the present invention aims at providing a coated cemented carbide tool whose both properties of a breakage resistance and wear resistance are improved and service life as a tool is lengthened.